Lithography is a technique used to fabricate semiconductor devices and integrated circuits. In lithography, a target substrate (usually a mask blank or semiconductor wafer) is coated with one or more layers of photoresist materials (resist). The resist is selectively exposed to a form of radiation, such as ultraviolet light, x-rays, electrons, and ions. The resist is then developed to remove part of the resist. The remaining part of the resist protects the underlying regions of the target. Regions from which the resist has been removed are subject to various additive (e.g., lift-off) or subtractive (e.g., etching) processes that transfer a pattern onto the target surface.
An electron beam or ion beam lithography system 110 (shown in FIG. 2) includes a charged particle (electron or ion) source 184 that generates a charged particle beam 116 directed through aperture plates 118, a blanking deflector 121, and focusing lenses 120 before reaching a final magnetic lens 112. Lens 112 further directs beam 116 onto a target 159 held on a target support 122 (also known as a stage). These lenses are electromagnetic or electrostatic, not light optic, structures. Charged particle source 184 generates the electron or ion beam. A control computer 123 controls the operation of lithography system 110.
One type of such lithography system is the variable axis immersion lens electron beam system, see, for example, U.S. Pat. No. 4,544,846, to Langner et al. (hereinafter “Langner et al.”), incorporated by reference in its entirety. FIG. 2 and FIG. 4A of Langner et al. are reproduced respectively as FIG. 1A and FIG. 1B of the present disclosure.
The variable axis immersion lens electron beam system includes deflection coils 43 and 45 (see FIG. 1A that depicts this structure in cross-section) that deflect an electron beam (shift the axis of the electron beam) so as to direct the beam to the desired location on the target 59. An immersion lens 12 includes one or more excitation coils 41 and 53 that generate a magnetic field when conducting an electric current (also called the focusing magnetic field). The focusing magnetic field has magnetic field lines that extend from a pole piece 13 to a pole piece 14 (FIG. 1B). The focusing magnetic field thus immerses a target 59 in an approximately uniform magnetic field (hence the name immersion lens) where the magnetic field strength is maximum near the surface of pole piece 14.
A deflection coil 11 generates a magnetic field (also called the deflection magnetic field) that shifts the magnetic axis of immersion lens 12 (hence the name variable axis) to coincide with the shifted axis of the electron beam. Deflection coils 11, 43, 45 vary the deflection magnetic field over time as the axis of the electron beam is shifted to scan target 59 during lithographic processes.
The varying deflection magnetic field creates eddy currents in electrically conductive system components downstream from deflection coil 11 (with respect to the direction of propagation of the electron beam), such as a target (wafer or mask blank) holder 16, a holder handler 20, and pole piece 14. Additionally, the varying deflection magnetic field may create eddy currents in a target 59 that is, e.g., a semiconductor wafer. The eddy currents in the above-described elements generate opposing deflection fields that deflect the electron beam, thereby creating placement error of the electron beam.
Accordingly, a disadvantage of the variable axis immersion lens is placement error caused by eddy currents generated by the deflection magnetic field. Alternatively, the system components subject to the deflection magnetic field can be of non-electrically conductive materials. However, the cost of the system increases with use of such materials. Thus, what is needed is a method and an apparatus that prevent the deflection magnetic field from radiating into electrically conductive components of the system downstream from the deflection coil, without adversely affecting the focusing magnetic field.